Machine tool



y 4, 1954 P. J. CAMPBELL 2,677,311

MACHINE TOOL Filed Nov. 2, 1950 9 Sheets-Sheet l May 4, 1954 Filed Nov. 2, 1950 MACHINE TOOL 9 Sheets-Sheet 2 v Q h...

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May 4, 1954 P. J. CAMPBELL MACHINE TOOL 9 Sheets-Sheet 5 Filed Nov. 2, 1950 Inave n20 May 4, 1954 P. J. CAMPBELL MACHINE TOOL 9 Sheets-Sheet 4 Filed Nov. 2, 1950 5 6 0 MW M a"? 0 0 E 9 a M? a 6 a 4 m 0 (r May 4, 1954 P. J. CAMPBELL MACHINE TOOL 9 Shets-Sheet 5 Filed NOV. 2, 1950 In we 2-2 2??? y 4, 1954 P. J. CAMPBELL 2,677,311

MACHINE TOOL Filed Nov. 2, 1950 9 Sheets-Sheet 6 [revere/3o? May 4, 1954 P. J. CAMPBELL MACHINE TOOL 9 Sheets-Sheet 7 Filed NOV. 2, 1950 May 4, 1954 P. J. CAMPBELL MACHINE TOOL 9 Sheets-Sheet 8 Filed Nov. 2, 1950 May 4, 1954 P. J. CAMPBELL MACHINE TOOL Filed Nov. 2, 1950 Patented May 4, 1954 UNITED STATES PATENT F F ICE MACHINE TOOL Paul J. Campbell, Middletown, Conn.

Application November 2, 1950, Serial No. 193,685

14 Claims.

This invention relates primarily to machine tools, and particularly to a duplicating or copying machine for reproducing in a workpiece the form of a model, template or pattern. The present application is a continuation-impartof'my prior copending application, Serial Number 65,052, filed December 13, 1948.

An object of thepresent invention is to provide improvements in apparatus and methodsfor causing a sensing element such as a follower or tracer to follow the contour of a pattern moved relative thereto, and for causing a tool associated with said follower to move relative to a workpiece in a path determinedoy the path of movement of the sensing element relative to'the pattern.

Another object is to provide improvements in methods and apparatus for relatively moving a pattern and follower and a workpiece and tool automatically with an electrically controlled scanning motion.

Another object is to regulate electrically the traversing motion of a pattern following element in accordance with changes in feed requirements of a tool controlled by said follower element.

A further object is to provide certain new and improved elements, and sub-combinations of elements, which though capable of use in other systems and in other machines, are-particularly useful in connection with the tracer-pattern scannin and following system and the toolworlcpiece scanning and feeding system of a duplicating machine tool.

Sther objects and. advantages of the invention will be apparent from the detailed description below of a presently preferred embodiment-of the invention and from the attached drawings which disclose the specific construction of said preferred embodiment, as follows:

Figure 1 a perspective view of a milling machine of standard commercial manufacture, but

modified for use as a duplicating machine by attachments incorporating tracer-pattern following mechanism, tool-workpiece feeding mecha nism, and pattern-tracer, workpiece-tool scanning mechanism constructed in accordance with the invention.

Figure 2 is'a transverse view, partly insection, of the duplicatingmachine of Figure 1.

Figure 3 is a side view, partly in section, of the tool and follower head of Figure 2.

Figure 4 is a. front view of the tool and follower head, together with theupper portion of the vertically adjustablelknee, thetransversely movable slide, and the longitudinally movablej table shown in Figures 1 and 2.

'2 Figure 4a is a "transverse sectional view of the micrometer "depth "stop.

Figure 5 is a longtiudinal sectional view through the tool and follower'he'ad.

Figure 6 is a top view, partly in section, of the tool and follower head, taken along theline '6---6 in Figure 5.

Figure 7'is a sectional view along the line 1-1 in Figure 5.

Figure 8 is a 'section'alv'iewalong the line 8-8 in Figure 5.

Figure 9 is a view along the line"9--9 in Figure 5.

Figure 10 is a rear view of a part of the vertical drive for the quill'anda section of the follower arm, with some sections of the housings broken away to reveal the gearing.

Figure 11 is a partial sectional view along'the line il-ll in Figure 7.

Figure 12 is a side view, partly in section, of the vertically adjustable knee, the cross slide, the table, and the cross slide feed and table feed drive mechanisms.

Figure '13 is a front view, partly in section, along the line l3 in Figure '12.

Figure 14 is va wiring diagram showing schematically the control circuits for the follower and tool feed mechanism, the cross slide and table feed mechanisms, andth'e interconnections therebetween.

Figure 15 is a partial wiring diagram showing schematically a modification of the control circuit for the tool and follower feed mechanism.

Apparatus embodying the invention can be constructed 'alternativelyas a special purpose machine or as anattachment'designed for use with a conventional machine tool of standard manufacture. In the embodiment illustrated in the drawing the invention comprises a specially constructed tool and follower head adapted to be used in conjunction with specially constructed cross slide and .table feeding mechanism as attachments on a standard milling machine of conventional design.

The conventional milling'machine parts consists of a base or frame 20 for supporting abiwheel 32 may be provided to rotate the arm about its horizontal axis. The end of arm 22 which projects over knee 24 is formed with a tongue 32 adapted to be telescoped and then bolted onto the yoke 38 of adapter 38 by bolt 40. Knee 24 is vertically adjustable on ways 42 with respect to the arm 22 and adapter 38. A screw jack 44 actuated by crank 43 is provided for effectuating such adjustment.

The conventional milling machine parts just described are fitted, according to the embodiment of the invention illustrated in the drawing, with a special combined tool and follower head attachment 48 bolted rigidly to the adapter 38, and with a special form of drive mechanism for moving the table 50 and the cross slide 52 longitudinally and transversely, respectively, relative to the tool and follower head attachment. The movements of the table, and consequently of the workpiece 54 and pattern 58 fastened thereto, are in a horizontal plane, or in a plane normal to the direction of movement of knee 24 on ways 42. Ordinarily, the angular adjustments of turret 26 and arm 22 are so made that when these parts are locked in place the tool 58 is fed toward and away from the work along an axis normal to the plane of movement of table 50; however, other angular settings of the head and tool relative to the table and workpiece may, of course, be used if desired. Once the head is locked in position the workpiece 54 and pattern 56 are moved simultaneously and equally during a machining operation longitudinally back and forth and also sideways, or transversely, with a scanning motion relative respectively to the tool 58 and follower 85 in a plane normal to the vertical Ways 42. This scanning motion is automatically controlled electrically at electrically selected rates or increments of cross or transverse feed, and with a superimposed control responsive to the higher rates of follower feed, by the circuit shown in Figure 14. This circuit controls both the drive motor 62 of the cross slide 52 and the drive motor of the table 56. As the table moves the workpiece and the pattern with a scanning motion under the tool and the follower, the follower and tool are both reciprocated alon parallel feeding axes whose angular position is determined by the setting of head 48. These tool and follower feed movements are effected by a motor 66 connected through infeeding (down) and outfeeding (up) electromagnetic clutches 68, I and thence through quill '12 to the tool spindle I4. Tool and follower feed clutches 68, F0 are controlled by the circuit of Figure 14 in such manner as to cause the follower 50 to follow exactly the surface contour of the pattern as the pattern moves thereunder, though in so doing the follower never actually touches the pattern, but is maintained at a substantially constant extremely small or minute spacing therefrom. Because the tool and follower are spaced at fixed distance apart on parallel axes and because they are interconnected by a follower arm 18 for simultaneous and equal feeding movements it will be apparent that the tool will be caused to describe a path relative to the workpiece exactly like that described by the follower as it follows the surface contour of the pattern, as the workpiece and pattern are moved by the carriage with simultaneous and equal scanning motions'respectively under the tool and follower.

Thus the tool 58, which has a cutting surface of the same size and shape as the pattern sensing surface of the follower (except that the follower 75 preferably is smaller by a dimension equal to said 4 constant small spacing) and which is separately driven for effecting metal removal by motor 29, will machine the workpiece to a surface of the same size and shape as that surface of the pattern scanned and sensed by the follower.

The specific construction of the tool and follower head attachment is shown in detail in Figures l to 11. This attachment provides for rotary cutting motion of the tool for metal removal, simultaneous and equal infeeding (down) and outfeeding (up) movements of the tool and follower relative respectively to the workpiece and the pattern, adjusting feed movements of the follower relative to the tool, and adjustable stop means for limiting the infeeding displacement of both tool and follower to a selected maximum depth.

The attachment is supported by the arm 22 to which it is rigidly fastened by adapter 38 bolted at 40 to the arm and at 82 to the generally tubular quill housing 84. The quill housing in turn serves to support the tool drive housing 8'3 secured to the top thereof, the follower housing 88 secured on the right side thereof (as viewed in Figure 5), and the clutch feed drive housing 96 secured to the left side thereof.

The quill housing has a cylindrical bore 92 in which the quill I2 is slidably, but non-rotatably mounted, for reciprocating tool feeding movements. The quill is fitted internally with antifriction bearings 96, 98 at each end thereof for rotatably supporting spindle I4. The inner bearing races are held in position on the spindle by a screw thimble I02 which locks the inner races of the upper and lower bearings and the spacer sleeve I04 therebetween firmly in place together, with the lower bearing race in abutment with shoulder I06 on spindle I4. A dust shield IDS, is provided at the upper end of the quill. The outer races of both lower bearings 98 are clamped together within the internal quill recess II2 by screw thimble H4 and the bearings are so designed that such clamping preloads the balls against the races to provide for high speed vibration-free operation. The outer races of upper bearings 96 are also preloaded for the same reason, but by a spring pressed loading ring H5 biased upwardly against the outer bearing races by compression springs II8 bearing against a washer I20 on shoulder I22 of a recess in the upper end of the quill bore. Both the upper and lower bearings are of the thrust resisting type, locking the spindle to the quill for longitudinal feeding movement therewith while permitting free rotation of the spindle relative to the quill.

The lower end of the spindle, which protrudes from the lower end of the quill, is formed with a tapered internal bore I24 for receiving a collet I26. The collet is drawn into the tapered bore by a drawbar I28 extending through the spindle and having its lower end screwed into the collet and its upper end seated in a recess I30 in the Y top of the spindle. When the drawbar is rotated in a tig tonin direction by a wrench placed in the socket I32 thereof collet H6 is drawn into the tapered bore I24 to force the collet jaws into tight driving engagement with the tool, in this instance an end mill 58.

The upper end of spindle I4 is formed with a circumferential series of axial or longitudinal splines I34 slidably, but non-rotatably engaged with like internal splines I36 at the lower end of an intermediate shaft I38 driven by pulley I40.

Intermediate shaft I38 is rotatably mounted in housing 84 by bearings I42, I44, the inner races of which are clamped together with the spacer sleeve I and thepulley I4Il'betWeen the shoulder I48 and nut 150 on shaft I38. The outer races of bearings I42, I44 are located within recesses in opposite endsof a bushing I56 clamped within the upper end-of the quill housing by aring I58 bolted to the housing at 160. A dust shield ring I62 is providedbelow the lowerbearing I44. Both bearings are preloaded by a loading ring I64 and compression springs I66 which act to bias the outer race of the upper bearin andthe inner race of the lower bearing upwardly with respectto their companion races.

Tool drive motor "80 is carried on the tool drive housing "85 which telescopesover and is fastened to the upper end of the quill housing 84. The motor drives intermediate shaft I38 through V pulleys I10, I40 connected by V belt I14. .Ajfiat pulley I16 on the motor shaft and a flat step I18 on pulley I40 are also provided, for high spindle speed operation. Power from the drive motor is transmitted'through pulley 'I 40 and'sh'aft I38 to spindle l lby splines I34, I36, regardless of whether the quill and spindle be "in a retracted position in which the shaft splines engage the lower end of the spindle splines or in an extended or protruded position in which the shaft splines engage the upper end of thespindle splines.

Quill 12 and spindle 14 carried thereby are extended out of the housing toward the workpiece or retracted into the housing away from the workpiece by the tool and follower feed motor 66 mounted on a bracket 18!] carried by the clutch housing 90 bolted to the left side of the quill housing (as viewed inFigure 5). Motor 66 drives a pinion I 32 through pulleys I84, I86 and a V belt I88. The pinion meshes with gear I90 on the drive housing I92 of upper clutch 68 (as viewed in Figure 5) and this clutch gear in turn meshes with a like gear 194 on the drive housing I96 of the lower clutch. Rotation of pinion I82 by motor 66 .causes the driving elements or housings I92, I96 of both clutches to turn at equal speeds, but in opposite directions ofrotation. Torque transmitted by the upper clutch 68 is applied to the splined driven clutch shaft I98 by the driven clutch plate 250 which is provided with internal splines engaging the externalsplinesof shaft I98. The shaft splines are formed in the shape of gear teethand therebyalso serve as a pinionfor driving layshaft gear 202 meshing therewith. A pinion 204 on a layshaft '20'6 driven by gear 202 engages a gear 261011 the quill drive shaft 208 and thereby drives the quill drive shaft pinion .gear 2I0. This pinion meshes with a rack 2J2 on the rear side of the quill and exerts .a thrust on the rack and quill tendin to move the quill axially or longitudinally in housing '84 to provide a tool and follower feed movement either toward or away from the workpiece and pattern depending upon the direction of rotation of feed .motor 66.

Torque transmitted by the lower clutch is similarly but oppositely applied to the rack and the quill. The torque is applied to splined shaft 2I4 by the clutch plate or driven element 2I6. The splined shaft serves as a pinion for driving the layshaft gear M8 on the'layshaft 220. Pinion 222 on the other end of the layshaftengages gear 224 on the lower quill drives'ha'ft 225. The lower quill drive shaft carries a pinion 228 meshing with rack 2 I 2 and exertingathrustthereon tending-to drive the'quil-l axially or longitudinally in housing 84 by the :lower clutch '10 upon rotation of teed motor 66. However, thelower clutch and its associated gear "train exert a thrust on the quill in the :opposite direction to thethrust exerted by the-upper olutchand its associated gear train. Thus the two clutches apply thrust simultaneously, butin opposite directions through separate or independent gear trains to the rack 212. If both clutches are continuously engaged regulated quill feed movements may be eifected in either of the two opposite directions merely by controlling the slip of one clutch relative to the slipof the other clutch. This enables a highly sensitive, fast acting and extremely accurate control of quill feed motion to be'obtained merely by varying't'he current flow through either clutch relative to the current flow through the other clutch. Backlash and play in the feed mechanism are constantly taken up because both drive mechanisms are continuously engaged, though in 'opposite'directions. A change in-the magnetic attraction between the driving and driven elements of either clutch is instantly reflected in a corresponding-change in the thrust force applied by said clutch to the quill; the two clutch-controlled thrust forces on the quill are continuously acting and there are no intervenin mechanical lags.

Both clutches are constructed in the same manner. A detailed view of the upper clutch is shown in Figure 11. The driving element is a cylindrical housing 592 of magnetic material such as iron rotatably mounted on-shaft 230 by bearings 232, 234 and containing'an annular recess for receiving a coil 236 wound around the axis of rotation of the housing. The high permeability path provided by the iron surrounding each cross-section of the coil is interrupted by an annulus 238 of non-magnetic material such as stainless steel, silver soldered within an annular gap 240 in the right hand end wall of the housing (as viewed'in Figure 11). Flux created by coil 235 attempting to bridge this gap passes instead into the highly permeable driven clutch element 208 and from thence back into the housing on the other side of the gap. The clutch plat 2% is thus attracted more or less strongly to the end face of the housing depending on whether more or less flux is passing around the gap 246 through the plate. The effect is that of a pair of annular electromagnetic poles having end faces separated by the annular non-magnetic gap 240 and bridged by the magnetic clutch plate 290. The holding force of the electromagnet on the clutch plate and therefore the frictional driving force of the driving clutch e1ement on the driven clutch element is varied as a function of the amount of flux created in housing I52 by coil 236 and this in turn varies with the current flowing through th coil. The slip of the clutch, or the speed of rotation of the driven element relative to the speed of rotation of the driving element, may therefore be regulated merely by controlling the current supplied to coil 236. Current is supplied to the coil by sprin contactors 242, 244 connecting th terminals of the coil to slip rings 246, 248 secured against an insulating plate 250 on the left side wall of the housing by an insulating wedge ring 252 and bolts 254. Brushes 256, 258 bearing respectively against the slip rings feed current to the rotating coil from the external stationary contacts 260, 2.62. It will be seen that in the construction as described most of the clutch mass is concentrated in the driving element, which continuously rotates with the feed motor 66 at substantially constantispeed. The variable speed clutch element or driven element is formed as a simple light iron clutch plate of low mass and hence low inertia, thus enabling inertia loads in the accelerating and decelerating portions of th mechanism to be kept to a minimum. The clutch plate is constantly attracted to the right side face of the clutch housing so that no reciprocating movement is involved in increasing or decreasing clutch slip; an increase in clutch current merely increases the attractive force between the driving and driven clutch faces and thereby increases the frictional forces therebetween to decrease the slip. To provide extreme smoothness of operation a porous paper lubricated sheet 234 (for example, a chemical filter paper of about .006 inch thickness, impregnated with a lubricant such as graphite is satisfactory for the purpose) is interposed between the opposed faces of the driving and driven clutch elements I32, 200. The clutch can then be operated continuously slipping, and to a degree closely regulated by the control of clutch current, without jerking, grabbing or chattering.

Follower arm 53 mounted in a housing 88 is attached to the quill 72 for reciprocating feeding movement therewith. The follower housing is bolted to the right hand side (as viewed in Figure of the quill housing 84. It contains a vertical shaft 260 fixed within upper and lower bosses 268, N0 on an axis parallel to the longitudinal axis of the quill. The follower arm i8 is slidably mounted on shaft 266 by sleeve bearings 2'52, 27 for movement with quill 2 to which it is connected by a key and slot connection 276, 228. The key 233 is screwed into a hole in the side of the quill and the head of the key projects snugly within the slot 2'18 of a socket member 28?; fastened to the left side of follower arm 78 by bolts 232, 283. The tubular housing 86 is cut away as shown at 284 to form an aperture enabling the follower arm to be connected to the quill as described and the aperture is elongated to permit longitudinal or reciprocatory movements of the quill and follower arm relative to th housing 86. Thus, quill movements are transmitted to the follower arm through the key 235 and socket member 280 as well as to the spindle 2'1, through bearings 96, 98, so that both the spindle and the follower arm move simultaneously and equally with each axial or longitudinal feeding movement of the quill.

The outer end of the follower arm is bifurcated as shown at 386, 288 to receive the follower shaft 280. This shaft carries a bushing 300 having a flange 362 around its upper end extending laterally beyond and resting upon the upper surfaces of the fingers 286, 288 on the end of the follower arm. The follower shaft is slidably mounted on a vertical axis within sleeve bearings 36 i, 305 at the upper and lower outer ends of the follower housing. Bushing 300 is siidably mounted on the follower shaft, being prevented, however, from movement upwardly along the shaft beyond a selected point determined by the position of the lower end of adjustable stop shaft 358. This shaft forms an upper stop pin 318 which extends through slots 3i2, in the follower shaft wall and which has its ends fastened in diametrically opposed holes in the bushing near the lower end thereof. Stop shaft 338 is fixed to the end of a micrometer spindle 3ft, adjustable vertically in the micrometer barrel Sis by rotation of micrometer thimbie 320. The micrometer barrel is press-fitted into the upper end of the tubular follower shaft 290 to provide a rigid or fixed connection therebetween. Weight of the shaft 290 (which slides freely through bearings 304, 306) is therefore transmitted to barrel 3 l8, to spindle 3 Hi, to stop shaft 308 and thence by way of pin 3i0, bushing 300 and flange 302 to the follower arm fingers 286, 288. Thus the weight of the follower shaft assembly causes flange 302 to remain in contact with the upper surface of fingers 286, 288 during both up and down movements of the follower arm, and the follower shaft is maintained in that position relativ to the quill determined by the adjustment of stop shaft 308. However, this is true only during normal operation, when the follower 60 itself takes no load; if due to some abnormal condition a load or force were imposed upwardly on the follower it would merely lift the shaft 290 and possible also the bushing 300, thus displacing the follower shaft upwardly relative to the follower arm and thereby providing a safety or release action which prevents damage to the follower and/or the pattern whose form it is sensing. Under ordinary conditions the weight of the assembly is entirely supported by flange 302 bearing on fingers 286, 288 but if for any reason an upward thrust is imposed on the follower shaft it is free to move upwardly if the thrust overcomes its weight, without restraint or hinderance by follower arm 18.

Two adjustments are provided for the follower shaft. The mechanical adjustment of stop shaft 306 by screwing spindle 3 l 6 in and out of micrometer barrel 3i8 enables fine adjustments to be made of the position of pin 3 l 0, and consequently of the vertical position of shaft 230, relative to follower arm I8. A second depth stop adjustment is shown at 322, 324 for electrically limiting the downward movement of the follower shaft and quill to a selected depth. Flange 322 secured to the top of shaft 230 has an electrode point 324 thereon which sparks to the upper contact surface of a micrometer screw 326 fastened to the follower housing 88 when the shaft 290 is lowered in the housing to a predetermined maximum depth. This depth is readily selected by rotation of micrometer nut 328 threaded on screw 326 and held longitudinally between two flanges 330, 332 fastened to the follower housing underneath the contact 324. When the nut is turned the screw is moved up or down through smooth cylindrical holes in flanges 330, 332 on opposite sides of the nut. At least one of these holes has a protuberance or key 336 on the wall thereof which fits into longitudinal slot 338 in the screw and thereby prevents rotation of the screw while permitting axial or longitudinal adjusting movement thereof, in response to rotation of the nut. A spring clip 340 is interposed between the lower flange 332 and the nut, to eliminate play and maintain a constant biasing thrust on the screw threads of the nut and screw. Electrical lead 342 is connected to the electrode point, and this lead transmits a signal when the point approaches within a very short predetermined distance from the screw, which is effective in a manner later to be described to prevent further feeding movement of the follower and quill toward the work beyond a depth determined by the said selected position of point 324 and screw 326.

Follower 60 is mounted within an insulating sleeve 346 within the lower end of tubular shaft 290. Compression bushings at each end of the sleeve are forced inwardly by tightening of the screw thimble 348 to grip the follower and thereby lock it in fixed insulated position relative to the shaft. The lower end of the follower projects downwardly beyond the lower end of shaft 290 by a distance sufficient to reach into the lowermost recesses or depressions in the surface of the pattern whose form is to be sensed. The upper end of the follower bears against contact point 349, which is riveted firmly into lining 362. The contact point is connected by very short leads 350, 352 to resistors 586, 604 which in turn are connected to lines 588, 606 leading to the control circuit of Figure 14. The follower, leads 350, 352, resistors 586, 604 and wires 588, 606 are all completely electrically insulated from the follower shaft and its associated parts by sleeve 346 and by an insulating lining 362 within the shaft above the sleeve. An insulating grommet 364 is provided where the wires 588, 606 pass through the shaft wall. A compression spring 366, which preferably has a rubber coating so that it serves also as insulation, forces lining 362 downward, thereby holding contact point 349 against the upper end of follower 60.

Table 50 to which both workpiece 54 and pattern 56 are fastened is movable in three dimensions. It can be adjusted relative to the tool and follower head 48 along a vertical axis by crank 46 which actuates the jack 44 to shift the knee 24 up or down on ways 42. Once adjusted to the proper vertical position in this manner the knee is locked in place by a clamp partially shown at 361 in Figure 1 and subsequent movements of the table are limited to a horizontal plane, either transversely on the cross slide ways 36 8 or longitudinally on the table ways 310. Lead screws 312 and 314 may be operated either manually or automatically to shift the cross slide and table respectively along the ways 368, 310.

Manual operation of the cross-slide lead screw 312, is effected by a crank 316 fixed to the screw and which can be turned by hand to rotate the screw in a ball thrust bearing (not shown) in housing 318. The outer race of the bearing is fixed to the knee and the inner race to the lead screw to prevent axial movement of the screw while permitting rotation thereof. Consequently, when the screw 312 is rotated the nut :380 threaded thereon is moved axially therealong, in one or the other transverse direction, causing the cross-slide 52 bolted at 38| to a bracket 382 int..- gral with the nut to slide transversely on ways 368. Automatic operation of the cross-slide lead screw 312 is effected by motor 62 controlled by the circuit of Figure 14. This motor, which is reversible to move the cross-slide in either direction, is drivingly connected to the screw through a speed reducing train comprising a motor pinion 384 which meshes with an intermediate gear 366 that drives a sprocket 386 connected through chain belt 390 to a sprocket 392 keyed to the lead screw. Motor 62 and the shafts on which the gears and drive sprocket of the reducing train are mounted are carried by a casing 394 fixed to the side of knee 24. An aperture 396 in the knee side wall is provided to enable the chain-belt to pass therethrough.

Manual operation of the table lead screw 314 is effected by a crank 398 fixed to the screw and which can be turned by hand to rotate the screw in ball thrust bearing 400. The outer race of this bearing is fixed to the table and the inner race is fixed to the screw to prevent axial movement of the screw relative to the table while permitting rotation thereof. The screw is threaded into a nut 402 fastened by bolt 404 to bracket 382 on the cross-slide nut 380. Consequently, when the table lead screw is rotated it is moved axially in one or the other direction through the nut 402 and causes table 50 to slide longitudinally on ways 310. Automatic operation of the table lead screw 314 is effected by table drive motor 64 carried by an apron 466 which depends from one end of the table 50. This motor, which is reversible to move the table in either direction, is drivingly connected to the table lead screw through a speed reducing train comprising a motor pinion 408 meshing with an intermediate pinion M2 connected to the lead screw drive gear 4M through an idler gear M6. The shafts M8, 420 on which the intermediate and idler gears are mounted are supported by apron 406.

Operation and control The pattern 56 to be reproduced is placed on table 50, and if desired fastened thereto in any convenient manner, in a position such that the longitudinal boundaries of the pattern lie within the limits of the range of relative longitudinal movement of the table and follower. Workpiece 54 is similarly positioned with respect to the tool and fastened in such position by clamps 422, 424 secured with bolts 42 6, 428. Thus, when the table is operated to cause the pattern to move longitudinally back and forth under the follower the workpiece will be moved identically back and forth under the tool. Both the pattern and workpiece are also within the range of transverse movement of the cross-slide relative to the tool and follower. Thus, when the table is shifted transversely by movement of the cross slide on ways 366 the workpiece and pattern will be moved identically under the tool and follower in a transverse direction. Any vertical adjustment that may be necessary is made by shifting knee 24 on ways 42 and then clamping the knee in an adjusted position in which the range of feeding movement of the cutting and sensing surfaces respectively on the tool and follower encompasses the vertical boundaries of the pattern surface to be reproduced.

The surface of revolution formed by the cutting edges of the tool should be of exactly the same shape and size as the sensing surface of the follower (except that the dimensions of the follower sensing surface should be smaller by an amount equal to the spacing maintained between the follower and the pattern). An accurate and simple way of accomplishing such result is to machine a hole in a metal block with the particular tool to be used and then to pour molten metal in the mold so formed to produce a casting in the shape of a tracer rod or stylus particularly adapted for finishing to size for use as a follower in combination with said tool.

When the pattern and workpiece have been properly positioned and fastened to the table, with tool 58 and follower 66 fixed in identical positions in the tool spindle 14 and follower spindle 290, respectively, the reversing lugs 43!), 432 are shifted to and then locked in those selected positions in the table side channel 434 at which the reversing switch 436 is actuated by the lugs at the predetermined or desired limits of longitudinal table movement.

The electrical connections to the cross slide motor, the table motor, the tool and follower feed motor, and the reversing switch 436 are, of course, so made that they will be operated in the correct sense to perform the functions described herein and illustrated in the wiring diagram of Figure 14. As shown in this figure, the

main control circuit for the whole machine comprises three interconnected component circuits having the following primary functions: a table feeds circuit indicated by the dotted outline 438 for controlling the horizontal longitudinal and transverse motions of the table and cross slide, a clutch control circuit indicated by the dotted outline 440 for controlling the vertical motion of the quill (and the tool and follower attached thereto), and a power supply circuit indicated by the dotted outline 442 for furnishing electrical current at the various potentials required. All three component circuits are energized from a single source of alternating current, for example, a 110 volt 60 cycle power supply line. A safety switch mechanism operated by a solenoid 444 across the power line is provided to guard against power failures, as shown at the left in Figure 14.

Once the set-up is completed the double pole single throw main switch 446 is closed, applying power from the main 110 v. 60 cycle supply line 448, 450 to the primary winding 452 of the power supply transformer and also to the motor supply line Y which is directly connected to the common terminal of the two reversing windings of the reversible table feed motor 64. Line Y is also similarly connected directly to the common terminal of the two reversing windings of the reversible cross slide feed motor 62. button 454 is then closed, energizing relay 456 to close switch 458 and apply power from the other main power line to the motor supply line Z. Once the switch 458 is closed in this manner it will be held closed by current passing through the shunt 460 until the main switch 446 is opened, unless some other interruption occurs in the supply of power which would also release the solenoid of the relay and cause the biasing means therein (not shown) to open switch 458 and prevent further current flow through line Z. This safety action prevents possible damage to the workpiece or pattern by accidental stopping and starting of the machining operation due to momentary power failures. If apower interruption occurs both motors 62, 64 not only stop, but cannot be restarted until the reset button 454 is deliberately closed.

When safety switch 458 is closed, alternating current at 110 v. 60 cycles is fed through line Z to the terminal 462, from whence a circuit is completed through relay 464 and reversing switch 436 connected in series across the lines Y and Z by leads 465, 466 and 469. When reversing switch 436 is closed the relay 464 is energized and the solenoid thereof is pulled downwardly against the biasing means (not shown) of the relay to move switch 470 to its lower position, and when the reversing switch 436 is open the relay is deenergized and the biasing means therein moves the switch 4'50 to its upper position.

The reversing switch preferably comprises a micro-switch actuated by the adjustable stop lugs 43B, 432 on the side face of the table. When the table reaches the end of its selected range of lon-' gitudinal travel to the right (as viewed in Figure 4) lug 430 strikes a cylindrical plunger 412 slidably mounted in a bore through bracket 4T4 secured to the end face of the cross slide, on which the bracket 414 is mounted. Thi exerts a thrust force on the plunger, causing it to slide through the bore. As the plunger is shifted axially in the bore by the thrust force so exerted thereon it is caused to turn by pin 476 which projects laterally from the plunger through a cam slot 418 in the* Reset side wall of the bore. This turning movement forces the outer projecting end of the pin downwardly against the operating lever 486 of the micro-switch and opens or closes the switch as the case may be. A like action occurs when the table moves to the left sufiiciently to bring the right hand lug 432 in contact with the other end of plunger 4'12, except that in this instance pin 416 is lifted by the cam slot from the switch lever 480 enabling the biasing means therein (not shown) to close or open the switch, as the case may be. At the end of one direction of table movement the micro-switch is closed, while at the end of the other direction of table movement the micro-switch is opened. In either case relay 464 is operated simultaneously with the opening or closing of the micro-switch, to move switch 410 to its alternate position.

Power from the terminal 462 of the motor supply line Z is also fed through switch 482 of a fast acting relay 484, actuated by the clutch control circuit in a manner later to be described, to a switch 486 operated by a sensitive relay 488. Switch 486 has two closed positions. In its upper position power from line Z (assuming switch 482 is closed) is fed through the leads 490, 492 (containing a manually operated switch 494) to the switch 410 of the 110 volt relay 464. The current then flows through lead 496 or lead 498 to one or the other of the two terminals of table feed motor 64, depending on whether the switch 410 is in its up position or its down position, respectively. The circuit is, of course, completed through the particular winding so energized as a result of its connection through the common terminal of both windings to power supply line Y. Thus, one or the other of the two windings of motor 64 is energized by current in phase with the power source. The winding which is not so connected to the power source is energized by an advanced phase current flowing in phase shifting condenser 500 and resistor 502. The motor is so wound that its direction of rotation depends on the relative phase of the currents in its windings. Therefore, when the reversing switch 436 is closed at one end of the selected range of table travel it will, by the resultant shift of switch 410, cause the table feed motor to reverse and thereby reverse the direction of table motion; when switch 436 is subsequently opened at t -e other end of the selected range of table travel it will by the resultant shift of switch 410 again reverse the table feed motor and cause the table to return again to its original direction of movement. This action, if it were not stopped by an interruption of power or altered by the action of the fast acting relay 484 or the sensitive relay 488, would continue indefinitely. The table would merely move back and forth along a single line between limits determined by the position of the adjustable lugs 430, 432, at a speed determined by the ratio of the reduction gear drive between motor 64 and table lead screw 314.

However, it is necessary in order to reproduce the form of a surface to have the follower pass over the whole of said surface, and the fineness or detail of the reproduction will be dependent to a large extent upon the number of surface points whose position is sensed by the follower. Therefore, a traversing motion effected by the cross slide is superimposed upon the longitudinal table motion previously described, preferably in such a manner that the relative motion of the table and follower (and of the table and tool) will be ascanning movement, for instance as; a series of very-closely spaced'paiallel. lines. This may be accomplished manually, by the hand crank 375 or by manual actuation of. the selector switch 554. It may also be accomplishedv automatically through the action of the sensitive relay 488.

Manual actuation of the selector switch 504 can be used to efiect manual control of the cross slide feed motor 62. When this selector switch is in its left hand position, connecting lead 506 with lead 508, power is fed from terminal 462 of line Z, through the switch 482 ofthe fast acting relay, lead 490, lead 556, selectorswitch 504, lead 558, reverse switch H), and lead 512 connected to one of the two reversing windingsof motor 62. The circuit is completed through lead 5|4 which con nects the common terminal of the motor windings to the power line Y, and the motor is caused to operate in a first direction of'rotation to cause the cross slide to be shifted transversely in a corresponding direction along the ways 358. Manual operation of the slide (and consequently the table carried thereon) in the opposite sense is effected merely by throwing the reverse switch 510 to its opposed or right hand position, in which lead 508 is connected to lead 5H5 was to reverse the relation of current phases in the respective motor windings and cause motor 62 to operate in the opposite direction of rotation. Motor 62 is of the conventional reversing type having two windings which can be energized out of phase with each other to cause the motor to operate in either direction of rotation. A phase shifting network comprising acapacitor 5l8- and a resistor 5253 may be provided for reversing purposes, in a conventional manner. It will be seen that manual operation of the cross slide in this manner, either mechanically by crank 376 or electrically by manually operated switches 594 and 5), can be effected either continuously or intermittently, at any time. The direction of transverse ieed can be selected by reversing switch 5| 0 when manually controlled electrical feed is being used, the duration of the feed being controlled by the time period during which switch 504 is held in its left hand closed position; The speed of transverse feed will, of course, be at a rate determined by the ratio of the speed reducing train connecting the cross slide lead screw with motor 62. This, assumes that cross'slide motor 62, like table motor 64; is a substantially constant speed motor, which preferably is the case.

Cross feed selector switch 504 canalso be placed in a right hand closed position and a vertical:

open position, as well as in the left hand closed position referred to above for manual electrical cross slide operation. When in the vertical or open position the switch completely cuts oil the flow of current to motor 62 and there can be no operation of the cross slide except mechanically by turning crank 3T5. Condenser 522, like condensers 524 and 526, are blocking condensers which permit no current ilow except momentarily when the switches are opened or closed to eliminate arcing at the switch contact points. Current can flow to lead 563 only through switch 504 and then only from lead 555 when the selector switch is in its left hand closed manual position, or only from leads 490, 528 through switch 48% when the selector switch 564 is in its right. hand closed automatic position.

In the automatic or right hand position of selector switch 504 power from line Z fed to switch 486 through lead 490 iscaused to flow 7 current level.

becomes charged the current flow either to the table motor 64 through lead 492 and switch 410 or to the cross. slide motor 62 through lead 528 and switch 504, andnever to both motors at once. When switch 486 is in its lower position the current from line Z is fed to the crossslidemotor 62 and when the switch is in its upper position the current is fed to the table motor 64. Thus, switch 486 actsselectively to operate either the longitudinal table feed or the transverse cross slidefeed, depending upon whether or not the sensitive relay 488 is energized.

Energization of relay 488 is effected in timed relation to the reversals of the longitudinal table motion by a time delay circuit including a twin triode vacuum tube 530 having two control grids 532, 534 connected through condensers 549 and 541. respectively to the upper and lower contact points of a switch 535 actuated by thesame arm 53! that actuates switch 470 of relay 454. Thus,

' when the reversing switch 436 energizes or deenergizes relay 4'64 switch 536 is moved to a lower orupper closed position, respectively, simultaneously with the switch 416. In its lower position, switch 536 connects the plus 350 v. direct current supply line 538 (supplied by the vacuum rectifier 540 and a filter circuit comprising condensers 5'42, 544 and inductance 556) through a condenser 54! to the right hand control grid 534 of the tube 530. In its upper position, switch 535 connects line 533 through a condenser 549 with the left hand control grid. 532 of tube 530. A resistor 548 is preferably included in the line 538.

The anodes or plates. 55B, 552 of tube 53:; are connected through. a resistor 554 to the 350 v.

' D. C. line 538. The cathodes 555, 553, are connected by leads 560, 562 to a line. 564 connected to ground through the solenoid winding of the sensitive relay 488. The two leads 5555, 558 leading respectively to condensers 54?, 5:39 are connected to ground through fixed resistors 57!], 512; the other sides of condensers 54?, 549 are also connectedto ground through variable resistors 5'54, 515. The tube is, of course, provided with a heater 518 supplied from a secondary winding 5883 on the power supply transformer in the usual manner.

It will be'seen that when switch 536 is suddenly moved by energization of relay 464 to its lower position direct current starts to fiow from line 538' through switch 536. to the condenser 54?. During the time the condenser is thus being charged, current flows through the variable resistor5-l4, creating a direct current voltage drop across the resistor in an amount that can be selected by adjustment of the variable contact 532.

Thiscauses the grid 534 to go positive relative to ground potential by a voltage equal to the drop across resistor 5M; resulting in an increase in the current flow between the right hand tube elements 558', 552 and through the line 554 and relay 488. The relay 488 is so designed and adjusted that it will hold. the switch 435 in its lower closed position whenever the current fiow through the line 554 exceeds a predetermined minimum The circuit is so designed that the increase in current flow through line 5134 resulting from a movement of switch 538 to its lower position raises the current flow through the solenoid winding of relay s88 above said predetermined minimum level and maintains said current fiow above said level for a time period dependent upon the' setting of the movable contact 582 of' the variable resistor 5M and upon the capacitanceof condenser 541. As condenser 55? through resistor 514 and consequently the potential applied to grid 534 becomes less and less until a point is reached at which the flow of current through the tube by way of leads 562, 554 drops below said predetermined minimum level, enabling the biasing means in relay 488 to raise switch 485 to its upper position. Finally, condenser 541 becomes fully charged and the grid 534 will be at ground potential. Though some current may still flow between tube elements 552, 558 under such conditions it will be below said predetermined current level and will be insuiiicient to energize the relay 48%.

Movement of switch 533 to its opposite, or upper, position similarly acts on the grid 532 to increase the current flow between cathode 556 and plate 550, during a time period determined by the setting of the movable contact 585 of the variable re istor 516 and by the capacitance of condenser 545. As a result the current flow through relay 488 by way of leads 560, 554 is increased above said predetermined minimum level during a portion of the period in which the condenser 545 is charging, and such energization of the relay maintains switch 486 in its lower closed position during said portion of the condenser charging period. As before, the length of time during which the relay 488 is energized so as to hold switch 486 in its lower position can be selected simply by varying the eiiective value of resistor 515 through adjustment or" movable con tact 584.

When the connection between line 538 and condenser 541 is broken by movement of switch 535 from its lower to upper position, condenser 541 will then discharge to ground through the resistor 510, whereby it will again be in condition to be charged as described above when switch 536 is returned to its lower position. Condenser 549 will be similarly discharged through resistor 512 when the connection from line 538 to condenser 548 is broken by movement of switch 530 from its upper to its lower position, whereby it will again be in condition to be charged as described upon return of switch 535 to its upper position.

Summarizing the operation of the table feeds circuit, motor 54 moves table 50 in a first direction when relay 488 is deenergized and switch 485 is in its upper position. Switch 436 will be closed upon completion of the table stroke, energizing relay 384 to move switches 410, 535 simultaneously to their lower closed positions. This at once connects the motor 64 for table operation in the reverse direction and prevents such operation by simultaneously breaking the circuit to the table feed motor through the movement of switch 435 to its lower closed position in response to energization of relay 488 as a result of the signal maintained on grid 534 of tube 530 during a predetermined time period immediately following the said closing of switch 536. While switch 486 is thus held in its lower closed position the motor 62 will be operated to feed the cross slide and table transversely in a direction determined by the position of switch 510. After a time delay predetermined by the setting of variable resistor 514 the current flow through relay 488 will have decreased sufiiciently to enable the relay bias to move switch 486 to its upper position. This breaks the circuit to the motor 62 (assuming switch 504 to be in its automatic or right hand position) and reestablishes the circuit to the motor 64. The result is that the table moves longitudinally to one end of its stroke, stops, is fed transversely by movement of the cross slide in a direction selected by switch 5l0 during a time period selected by the variable resistor 514, and then is again moved longitudinally but in the opposite direction. A like sequence of events occurs at the other end of the stroke except that there the switch 436 is opened rather than closed and except that the traverse control is through variable resistor 516 rather than resistor 514. Thus, the follower and tool are caused to scan the surfaces of the pattern and workpiece, by relative movement thereof back and forth in a hori zontal plane along parallel spaced lines, the spacing of which lines can be varied at will by ad justment of resistors 514, 516. Because the se tings of variable resistors 514, 516 may be separately adjusted, the transverse table feed at the end of each longitudinal table stroke may be varied independently of the transverse table feed at the end of the opposite table stroke. As before stated, the speed of cross slide motion is preferably constant; therefore the amount of transverse feed or the distance through which the cross slide moves transversely at the end of a table stroke will be determined by the time period or duration of cross slide movement. This time period can be established between zero time for the setting of contact 582 or contact 58 3 in which a minimum resistance value is interposed between ground and the grid side of condenser 541 or condenser 5 19, up to a maximum time period for the setting of contact 582 or contact 584 in which a maximum resistance value is interposed between ground and the grid side of condenser 541 or condenser 545.

As the follower and tool are caused to scan the respective surfaces of the pattern and workpiece the clutch control circuit causes the follower to move vertically at rates of motion in both directions of travel necessary to cause the sensing surface of the follower to be maintained at a very small substantially constant selected spacing, or gap, from the surface of the pattern. For example, the clutch control circuit may be designed and adjusted to cause the follower to be maintained at a constant spacing of about .0015 inch from the pattern surface, for a machine of the I type shown in the drawing. It can, of course, be

designed and adjusted to maintain other predetermined constant spacings. For instance, for large machines it can be set to maintain a constant spacing of about .003 inch and in smaller machines a spacing of about .0005 or even less. Where larger constant spacings of a value greater than about .003 are desired a higher voltage power source for creating the follower signal preferably is used.

Clutch control circuit 440 operates on the same rinciple as the clutch control circuits shown and described in my prior application S. N. 65,052. filed December 13, 1948. The sensing surface of the follower 60 is connected through a resistor 586 by leads 588, 590 to the +1000 v. D. C. line of the power supply circuit 442. This D. C. line is supplied with power by the vacuum rectifier 592 and a filter circuit comprisin condensers 594, 598 and inductance 598. Pattern 56 is grounded, as shown at 600, so that at open cir-- cuit (no spark or current flow between follower 60 and pattern 55) a D. C. potential or" approximately 1000 v. is continuously maintained across the gap between the follower and pattern. Upon approach or" the follower into sufficiently close proximity with the pattern the air gap therebetween is broken down by the 1000 v. D. C. potential and a spark is established therebetween. It

is believed that the air in the gap between the follower and the pattern becomes ionized as the gap breaks down and current starts to flow-and thatthe current of the spark is carried bythe ionized air. Whatever the reason, it'has been found that once the spark is established (which usually occurs with a construction such as illustrated in the drawing at slightly more than .003 inch) it will be maintained substantially continuously with a current flow across the gap that varies as a function of gap length. Because of the relatively large value of the resistance 58.6 such current flow never reaches a very. large value-it is still of the order of tens of micro..- amperes even under short circuit conditions, i. e. when the follower is deliberately placed. into actual physical contact with the pattern Consequently the discharge across the gap is under all operating; conditions a spark type discharge ofa small cur ent at high voltage across a short air gap, with no 1. citing or fusing or burning such might occur with large currents The pattern follower surfaces can therefore-be compose-:1 of almost conductive material. A metal. follower has been found highly satisfactory, for instance of steel, brass, or low fusing non shrinking lead or bismuth alloys. The -pattern, for example, may be merely a moist plaster of Paris; a modeling or casting wax coated or covered. with an electrically conductive paint, paste or liquid; or it toomay be composed of metal.

Asthe current flows across the gap between the follower and the pattern a voltage drop is produced across resistor 586 which varies with said current flow. 'Zhereforea signal potential is established on the follower which will vary as a function of the length of the spark, or function of the air gap. between thefollower and pattern at their points of closest proximity. This signal potential is transmitted by lead 1302 through resistor 5G4 and lead 606-t0 control grid. 6:38 of the triode vacuum tube. till, lch is connected in a cathode follower circuit including resistor cathode 6M. and anode 65G. The tube con us a heater till supplied from. a secondary winding 02a; of the power. supply transformer the usual manner. Cathode 6M is connected to ground through the resistor: 6 i2, and anode oil; is connected to the +1000 v. D. 0. line 593 through lead 622.. With. this arrangement, a change in the potential on grid 608, such as would result from a signal trans1nitted from follower BE through lead 602 and resistor 88s to the grid, results in a corresponding change in the current flow through the tube between the cathode and anode thereof. This in turn creates a corresponding change in the voltage drop acr ss resistor 2. The effect is to produce a signal in lead 625 (connected to the cathode) whose voltage is a function of the initiating signal. produced on. the follower, but whose current is at a higher level. The voltage onthe cathode of tube Bill is transmitted through lead 626 a condenser the other side of which is connected through lead @353 and resistor G ill to ground. As the voltage on the cathode of tube varies, due to varying current flow through the tube, the charge on the plates of condenser i328 tends to vary accordingly, and current therefore flows between the condenser and ground through resistor 540 at a rate proportional. to the rateoi' change of charge in condenser 328. The voltage drop. across resistor E40 in: the current. flowing in resistor 18 due to current flow between condenser 628 and ground thus. causes. the voltage. on lead 638 to be substantially proportional to the rate of change of voltage onthe cathode of tube Bill.

The voltage on lead is applied to a second condenser 63$, the other side of which is con.- necte. through lead 561% and resistor 5 12. and lead 62 to the adjustable contact 523 on .resister As the voltage on lead 633 varies, the charge on. the plates of condenser tends to vary accordingly, current flows through lead 5 -34 and resistor at a ratewhichis substantially proportional. to the rateof. change of charge on the plates of the condenser and also pro.- portional, re, to the of change of voltage onv lead 633 between condensers E28 and 638. It will be seen that, since the voltage on lead is proportional to the rate of change of voltage on the cathode of tube till, then the current flowing in lead 53 2 and resistor 642. must substantially proportional to ti e rate of change of cathode voltage on tube ti Furthermore; since the cathode voltage of tube is substan: tially proportional to the length of gap between follower S 3, and pattern tb nthc current flowing in lead 55% and resists 32 is substantially proportional theacceleration of the length of saidgap.

The current which flows. in lead 644 and re.- sistorfidE- also. flows through lead and adjustablecontact and through that portion of resistor s52 between contact and ground.

The voltage on lead 565 is equal at any instant to. the: algebraic sum of the voltage drop due to the flow of. cathode current from tube Hi0 through that portion of resistor 6|2 between contact 623. and, groundplus the voltage dropdueto the. current. which is flowing through resistor 542 and that portion of resistor 6l2 between contact623 and ground; Since the cathode current intube 6 I @is a function of the length of gap between the pattern and follower, and since theourrent in resistor 542 is a function of the acceleration of said gap, then the voltage on lead fi llls at. any instant a function of both the length and. the acceleration of the gap between follower t0 and. pattern 56.

The voltage on lead644 is transmitted through leadz632. tothe control grid 634 of triode vacuum tube .636' The anodeyfiEfi of tube 636 is connected. through resistor 64.6 andlead 648-to the plus 350 volt D. C. line of the power supply cir-, cuit 4'42 and the anode is also connected through lead 558 and resistor GEO-to the minus 350 volt D. C. lineof the power supply circuit. The plus 350 volt D. C. power is preferably supplied in the usual manner byvacuuni rectifier tube 540 and a filter circuit comprising condensers 542, 5 and inductance 5416. Similarly, the minus 350 volt D. 0. power preferably is supplied by vacuum rectifier tube 662 and a filter circuit comprising condensers 8'54, .656 and inductance 668. The cathode of tube 535 is connected through a resistor 65s to ground and. the cathode is also connected through lead 510 and resistor 812 to the minus 350; volt D. C.. line of power supply circuit 442..

Achangein the potential on the grid 634 of tube 636, such aswould-result from a change inthe voltage. in lead 632 connected thereto, causes a corresponding change in the current flowing through thextubebetween the anode :and the cathode thereof. Such a change in current flow in the tubecauses a corresponding change 645 and resistor @553 connected respectively to the anode and cathode of the tube. The resulting voltage drops across these resistors appear as changes in the potentials on the anode and cathode of tube the cathode voltage changing in the same direction as the applied grid voltage and the anode voltage changing in the opposite direction. For example, an increase in the voltage on grid tS l, such as would result from an increase in the gap between follower 60 and pattern 56, would cause an increased current flow through tube 636 with a corresponding increase in voltage on cathode B52 and a decrease in voltage on anode 655.

A change in the voltage on anode 656 and on lead 658 connecting the anode to resistor 660 causes a corresponding, but smaller, change in the potential on contact 614 on resistor 680. Similarly, though in opposite phase, a change in the voltage on cathode G52 and on lead 610, connecting the cathode to resistor 612, causes a corresponding, but smaller, change in the potential on contact 376 on resistor 612. Resistors 668, T2 preferably have high values of resistance relative to resistors 643 and 65B so that current flowing them will have little influence on anode and cathode potentials in tube 836. The positions of contacts 874, 610 preferably are so chosen on their respective resistors 680, 512 that during operation of the machine, their potentials will vary within approximately equal ranges slightly below ground potential.

The potential of contact 674 is transmitted by lead 538 to one control grid 680 of twin triode vacuum tube 532, where it controls the flow of current between anode 696 and cathode 688. Similarly the potential of contact 516 is transmitted b lead 684 to the other control grid 686 or" tube where it controls the current flowing between anode S98 and cathode 690. Both cathodes are connected to ground by leads 692,

Anode S95 is connected in series with milliarnmeter and the outfeeding electromagnetic clutch to lead E92; anode B98 is connected in series with milliainnieter I06 and infeeding electromatic clutch 68 to lead 102, which connects the common side of both clutches through the coil of fast acting relay 484 and lead 648 to the plus 350 volt D. C. line of power supply circuit 5&2. As the potential of grid 680 varies, due to variation in the voltage on contact 6'14 connected thereto, a corresponding change in current occurs in the series circuit between lead E02 and ground, comprising the space between cathode tee and anode B98, milliammeter Itil, and clutch iii. For example, an increase in voltage on contact 3 and grid 680 connected thereto such as would result from a decrease in the gap between follower st and pattern 56, causes an increase in the flow of current from lead 102 through outfeeding clutch i0, milliammeter I00, and across the space between anode 596 and cathode to ground. Conversely, a decrease in voltage on grid sac, such as would be caused by the follower moving away from the pattern, causes a corresponding decrease in current flow through outfeeding clutch l0 and milliammeter "its; and by proper selection of the position of contact Sit on resistor 660, the current flow through clutch is and milliammeter H10 preferably is reduced to zero when the follower moves beyond a certain predetermined distance from the pattern.

In a similar manner, but in opposite phase, a variation in the potential on grid 686 of tube 682 due to variation in the potential of contact 610 connected thereto causes a corresponding variation in the current fiow through infeeding clutch 0S and milliainineter 706. For example, a decrease in the voltage on contact 616, such as would result from a decrease in the gap between follower G?! and pattern 56, causes a decrease in current fiow through infeeding clutch 68 and inilliamnieter I86; and by proper selection of the position of contact file on resistor 612 the current in clutch 63 preferably is reduced to zero when the follower approaches the pattern nearer than a certain predetermined minimum space. Conversely an increase in the voltage on contact 6'15 such as would be caused by an increase in the gap between the follower and pattern causes an increased current to flow in inieeding clutch 88 and milliammeter 10S.

fhe total current flowing in both clutches 68, it passes from the plus 350 volt D. C. line of the power supply circuit through lead 648, through the coil of fast-acting relay Q84, and lead (02, from which it divides and flows in various proportions to the two clutches, depending on the relative potentials of control grids 68%, 686. Relay 484 operates to open switch 482 when the current in its coil exceeds a certain predetermined value and releases to allow switch 482 to be closed by its biasing means when the coil current decreases below said predetermined value. Since switch 482 connects both feed motors 62, E i to the power source Z, neither of these motors can operate when switch 482 is open. Therefore, when the current in the coil of relay 484 exceeds a certain predetermined value either motor 62 or (whichever one happens to be running) will stop and remain stopped until the relay coil current decreases below said predetermined value. Motors 52, 84 are preferably of th synchronous inductor type, which is capable of stopping with extreme rapidity upon interruption of the power source to which it is connected. The current flowing in the coil of relay 4134 is at a minimum when the gap between follower and pattern 56 is at its preselected mean value. If the gap is either increased or decreased, within the normal operating range, from said preselected mean position the current in one clutch increases slightly and the current in the other clutch decreases by an approximately equal amount, reaching zero at either the high or low limit of the normal operating range. (Although current in the clutch reaches zero the clutch driven element 200 remains lightly attracted against the driving element because of the residual magnetic flux therein.) In the circuit herein described the preselected mean gap length is approximately .0015 inch, and the normal operating range is plus or minu 0-305 inch; that is, in normal operation the ice between follower S0 and pattern 56 at the point of closest proximity varies between lin ts of approximately .001 inch and .002 inch. Within this range the current flowing through the coil of relay 4&4 is substantially constant because any increase in current flowing to one clutch is offset by an approximately equal decrease in current flowing to the other clutch.

ever, i; the gap between the follower and pattern is varied beyond the normal operating range the current in one clutch remains at zero while the current in the other clutch continues to increase, resulting in an increase in the current in the coil of relay 484 to a value above the level required to open switch 482 and thereby stop 212' feed motor as or 64, whichever one happens to be running.

It will be seen that a small deviation of the gap length between follower and pattern to either side of the selected mean gap length causes a biasing of th in the clutches which results in an increa e in the torque delivered by one clutch and a decrease in torque delivered by the other clutch. The net thrust delivered to the quill by the clutches, so biased is in a direction tending to reduce the deviation which initiated the biasing of the clutches. If the clutch drive succeeds in correcting the deviation or at least prevents it from increasing beyond the normal operating range, the table and cross-slide continue their feeding and scanning action. ever, if the deviation increases beyond certain predetermined limits, the clutch biasing not only increases, but the motion of the table and cross slide immediately stop clue to the opening of switch 482 relay 8s, and remain at rest until the deviation from the mean of the gap between follower and pattern has decreased to a small enough value to allow switch 482 to be closed.

Because of the gap-acceleration signal superimposed upon the gap-length signal by the differentiating circuit comprising condensers @328, 6-36 and resistors the relative currents fiowing in clutch s 68, iii are a function not only of the length of between follower 6t and pattern but of the acceleration of the gap length. et iod of controlling clutch currents by a co ration of position and accelerasi ix. nits in a very srnootl following action without steps, overshooting or stability. Without the superii iposed acceleration signal the clutches we d a net restoring force on the quilland follower arm which would be substantially proportional to the deviation of the gap length between the follower and pattern from the selected mean, regardless of the rate at which the deviation is changing and it would be difficult to obtain a smooth following action. However, when an acceleration signal is superimposed upon the position signal, the clutches react not only in proportion to the deviation of the gap length from the selected mean, also to the acceleration of the l? example, follower might be exactly at its mean position relative to the pattern, but accelerating away from it at a high rate. Under his condition a high current would flow in the infeeding clutch, tending to prevent any deviation the follower outwardly from its mean position.

It has been found to be advantageous to provide a means or automatically backing the cutter and follower away from the workpiece and pattern in the event of power failure in the power lines M8, see. Control grid @80 of tube 682 is connected by lead Hill through switch 136 to ground. Switch held in the open position loo by the energized coil i l l of relay #156 during operation of the control circuits. In the event of power failure in the supply lines M8, 45% coil 444 is immediately de energized, thereby permitting the biasing means of the relay to move switch 136 to its closed position. The potential of control grid 680 is thereby immediately raised to ground potential causing a relatively large current to flow through outieeding clutch ill during the period while the clutch-drive motor 55 is coasting to a stop. The filter condensers 542, 5440f the 350 volt supply are of large enough capacity to furnish current to the upfeeding clutch during the stopping period. The cutter and follower are thereby backed away from the workpiece and pattern in case of power failure and the danger of damaging the workpiece or pattern due to improper following is avoided.

Milliamineters its conected in series respectively with clutches Ill, til give a visual indication of the current flowing in. the respective clutches. Such an indication is usefulin making the initial adjustments of the circuit, such as the settings of contacts The inilliarnmeters are also useful in setting up the pattern on the machine because their indications of rela-- tive currents flowing through the clutches serve as an indirect indication of the distance of the follower from the pattern surface, enabling very precise adjustments to be of the height of the follower relative to the tool.

To limit the downward feeding of the cutter and follower, as in making roughing cuts, a micrometer stop contact point are connected gap therebetween, causing a current to flow inresistors 694, Hi], through the spark gap to ground. The resulting voltage drop in resistor 604 causes a decrease in the potential on grid 608 of tube Bill; and by the previously described action of the clutch control circuit in response to the voltage on grid 698, further down feeding is prevented even though the surface of pattern 56 may recede from follower When an upward sloping surface of the pattern approaches within the normal operating distance from the follower, the gap bSJWdBIl the follower and pattern will again take over control of the clutches, and the follower will follow the contour of the pattern. The micrometer depth stop thus serves to limit the maximum depth of cut to any predetermined value within the capacity of the cutter.

In order to redu e reactive oscillations of the spark between follower Bil and pattern 56 or between contact 324 and micrometer screw 326 to a negligible value, resistors Hill preferably are of relatively high resistance and preferably are located as near their respective spark gaps as practical.

An alternative signal input circuit is shown in Figure 15. In this circuit a trio-dc vacuum tube H4 is connected. in a cathode follower circuit comprising tube lid and resistor i l t. The anode H8 is connected to the plu icoc volt D. C. line by lead 50%. The cathode "its is connected to ground through resistor it. The cathode is also connected through series resistors 22 i, 2 2% to an electrode 122 of a spark gap. (The electrode 122 may be either a follower, or a micrometer depth contact.) The grid 523 of tube lid is connected by lead 730 to lead 532 between resistors i2 6. When electrode "522 is far enough away from a ground member such as Mil, that no spark exists therebetween, no current flows in resistors 724, 726; and grid potential is therefore equal to cathode potential. Under this condition relatively large current flows through tube lie between the anode and cathode thereof and through resistor 1| 6. Because of the resulting relatively high 

